A black hole represents a region of spacetime where gravity is so intense that nothing, not even light, can escape its grasp. This extreme gravitational field arises from the gravitational collapse of matter, typically the core of a massive star. The idea of a black hole “collapsing” usually refers to three distinct events in their life cycle. The first is the formation process, where the precursor star collapses to create the black hole. The second relates to the black hole’s eventual, slow disappearance through a quantum process called evaporation. The third and most energetic event is the collision and merger of two black holes into a single, larger entity.
The Stellar Collapse that Creates a Black Hole
Black hole formation is the end-stage of a massive star, typically one starting with at least eight times the mass of our sun. During its life, the star maintains hydrostatic equilibrium, balancing the inward pull of gravity with the outward pressure generated by nuclear fusion in its core. This fusion converts lighter elements into heavier ones, providing the energy that prevents collapse.
This balance breaks when the star’s core fuses all lighter elements into iron. Iron cannot release energy through fusion; fusing it would consume energy, halting the outward thermal pressure. With the power source extinguished, gravity initiates a rapid, inward collapse of the core. The core plunges inward until its density crushes atomic structures, forcing protons and electrons together to form neutrons.
This sudden halt of the core collapse by neutron degeneracy pressure creates an outward-moving shockwave, which ejects the star’s outer layers in a Type II supernova explosion. The fate of the remaining core depends on its final mass. If the core mass exceeds the Tolman-Oppenheimer-Volkoff limit (around 1.5 to 3 solar masses), the pressure of the neutrons is insufficient to resist gravity.
The collapse continues unchecked, compressing the core to a point of infinite density called a singularity. As the core shrinks, its gravity concentrates until the escape velocity approaches the speed of light. The boundary where the escape velocity equals the speed of light is the event horizon. Once the core passes inside this boundary, also known as the Schwarzschild radius, the black hole is formed, and the gravitational collapse is complete.
The Slow Disappearance of Black Holes
While nothing can escape the event horizon, a black hole is predicted to slowly disappear through a process known as Hawking Radiation. This theoretical mechanism arises from the interaction between quantum mechanics and the black hole’s gravitational field near the event horizon. Quantum field theory suggests that empty space is constantly filled with “virtual” particle-antiparticle pairs that spontaneously appear and quickly annihilate each other.
Near the event horizon, the extreme curvature of spacetime can separate one of these virtual pairs before they recombine. One particle may cross the event horizon and fall into the black hole, while its partner escapes, appearing to an outside observer as a stream of thermal radiation. To conserve total energy, the particle that falls into the black hole must carry a negative energy value.
The absorption of a negative-energy particle reduces the black hole’s mass and rotational energy, leading to a gradual loss of mass. This slow emission of particles is Hawking Radiation, meaning black holes radiate like a black body with a specific temperature. The temperature is inversely proportional to its mass, so smaller black holes are “hotter” and radiate energy at a greater rate than larger ones.
For a stellar-mass black hole, the process is incredibly slow, requiring timescales that dwarf the current age of the universe. As the black hole loses mass, its temperature rises, and the rate of evaporation accelerates exponentially. The final moments of the black hole’s life culminate in a burst of high-energy radiation as the remaining mass vanishes.
Collisions Between Black Holes
The most energetic event involving black holes is their interaction and merger. When two black holes are gravitationally bound in a binary system, they orbit each other, steadily losing energy by emitting ripples in the fabric of spacetime. This loss of energy causes the two objects to spiral inward toward each other in a process called inspiral. As they get closer, the rate of energy loss increases dramatically, accelerating the spiral.
In the final fraction of a second before the collision, the speed of the black holes approaches a significant fraction of the speed of light, and the system emits an extraordinary torrent of gravitational waves. This merger is one of the most powerful events in the universe, briefly radiating energy equivalent to all the light from all the stars in the observable universe.
The two event horizons eventually merge, forming a single, larger black hole. The newly formed black hole quickly settles into a stable, spherical shape, radiating away any remaining excess energy as a final burst of gravitational waves called the “ringdown.” This entire process, from the final inspiral to the ringdown, confirms Albert Einstein’s 1916 prediction of gravitational waves.
Observatories like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and its partner Virgo have detected numerous examples of these collisions, measuring the distortions in spacetime as the waves pass through Earth. The first confirmed detection in 2015 involved two black holes that merged, converting a portion of their combined mass entirely into gravitational wave energy. These merger events provide astrophysicists with insights into the properties of black holes and the nature of gravity under extreme conditions.